Leetcode 236

来源：互联网发布：centos minimal 区别编辑：程序博客网时间：2024/05/16 23:45

Given a binary tree, find the lowest common ancestor (LCA) of two given nodes in the tree.

According to the definition of LCA on Wikipedia: “The lowest common ancestor is defined between two nodes v and w as the lowest node in T that has both v and w as descendants (where we allow a node to be a descendant of itself).”

        _______3______       /              \    ___5__          ___1__   /      \        /      \   6      _2       0       8         /  \         7   4

For example, the lowest common ancestor (LCA) of nodes 5 and 1 is 3. Another example is LCA of nodes 5 and 4 is 5, since a node can be a descendant of itself according to the LCA definition.

class TreeNode {    int val;    TreeNode left;    TreeNode right;    TreeNode(int x) {        val = x;    }}public class Solution236 {    /*public boolean dfs(TreeNode root, TreeNode p, Stack<TreeNode> tmp) {        *//*if(root == null || root == p) {            if (root == null) return false;            else {                tmp.push(root);                return true;            }        }        tmp.push(root);        if(dfs(root.left,p,tmp) || dfs(root.right, p, tmp)) {            return true;        }        tmp.pop();        return false;*//*        if(root == null) return false;        if(root == p) {            tmp.push(p);            return true;        }        tmp.push(root);        if(dfs(root.left, p, tmp) || dfs(root.right, p, tmp)) {            //tmp.push(root);            return true;        }        tmp.pop();        return false;    }    public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {        Stack<TreeNode> queuep = new Stack<TreeNode>();        Stack<TreeNode> queueq = new Stack<TreeNode>();       // dfs(root, p, queuep);        dfs(root, q, queueq);        *//*TreeNode result = null;        for (int i = 0; i < queuep.size() && i < queueq.size(); i++) {           if(!queuep.get(i).equals(queueq.get(i))) {               break;           }            result = queuep.get(i);        }*//*        Enumeration<TreeNode> itemsq = queueq.elements();        while (itemsq.hasMoreElements()) {            System.out.print(itemsq.nextElement().val+" ");        }        return null;        //return result;    }*/    public boolean dfs_s(TreeNode root, TreeNode p, Stack<TreeNode> tmp) {        if(root == null || root == p) {            if (root == null) return false;            else {                tmp.push(root);                return true;            }        }        if(dfs_s(root.left,p,tmp) || dfs_s(root.right, p, tmp)) {            tmp.push(root);            return true;        }        return false;        /*tmp.push(root);  //如果用这种方法虽然结果是正确的，但是会发生栈溢出，        if(dfs_s(root.left,p,tmp) || dfs_s(root.right, p, tmp)) {            return true;        }        tmp.pop();        return false;*/    }    public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {        Stack<TreeNode> queuep = new Stack<TreeNode>();        Stack<TreeNode> queueq = new Stack<TreeNode>();        TreeNode result = null;        if(dfs_s(root, p, queuep) && dfs_s(root, q, queueq))        {            for (int i = queuep.size()-1, j=queueq.size()-1; i >=0 && j >= 0; i--, j--) {                if(!queuep.get(i).equals(queueq.get(j))) {                    break;                }                result = queuep.get(i);            }        }        return result;    }    public static void main(String[] args) {        TreeNode p = null, q = null;        TreeNode root = new TreeNode(3);        root.left = new TreeNode(5);        p = root.left;        root.left.left = new TreeNode(6);        root.left.right = new TreeNode(2);        root.left.right.left = new TreeNode(7);        root.left.right.right = new TreeNode(4);        root.right = new TreeNode(1);        //q = root.right;        q=root.left.right.right;        root.right.left = new TreeNode(0);        root.right.right = new TreeNode(8);        Solution236 ans = new Solution236();        TreeNode result = ans.lowestCommonAncestor(root, p, q);        //System.out.println(result);    }}

0 0